FIG. 32 illustrates a block diagram of a conventional prior art Y/C separator widely used in video tape recorders, TV receivers and other equipment. In the prior art diagram, an input terminal 1 is for input of the Y/C composite video signal. A bandpass filter 2 is for separating a signal of the color carrier signal band, a 1 H delay line 3 is for delaying the color carrier signal by a 1 H period, and a subtractor 4 is for subtracting the output of the 1 H delay line 3 from the output of the bandpass filter 2. An output terminal 5 is for outputting the color carrier signal output, a delay device 6 is for delaying the signal, and a subtractor 7 is for subtracting the output of the subtractor 4 from the output of the delay device 6. An output terminal 8 is for outputting the luminance signal.
The bandpass filter 2 extracts only the video signal in the color carrier signal band from the Y/C composite video signal via the input terminal 1. This goes to the 1 H delay line 3 and the subtractor 4. The signal is delayed by the 1 H delay line 3 by a 1 H period equivalent to one horizontal scanning line and supplied to the subtractor 4.
In the NTSC system color signal, in order to avoid mutual effects between the luminance and color carrier signals, the relationship between the color carrier signal Fsc and horizontal sync signal fH is set at EQU Fsc=(fH/2).times.455=approx. 3.58 MHz
The color carrier signal is overlapped on the high band component of the luminance signal. When this relationship exists, the color carrier signal whose polarity reverses every frame mutually cancels. Also, in the correlation between adjacent scanning lines, phase is reversed and is averaged, thus the effects of the color carrier on the luminance signal become indistinguiushable.
However, since line correlation is strong between mutually adjacent scanning lines, the luminance signal waveform remains substantially the same, but the color carrier phase reverses, as mentioned above.
This relationship is maintained even between the two signals input to the subtractor 4.
The subtractor 4 yields only the color carrier signal component through the process (Y+C)-(Y-C)=2C. For the sake of simplicity of explanation, it is assumed that this is attenuated 178 to produce output C.
The color carrier signal separated by the subtractor 4 is sent to both output terminal 5 and the subtractor 7.
The delay device 6 functions to adjust the timing between the Y/C composite video signal from the input terminal 1 and the separated color carrier signal from the subtractor 4, at the inputs of the subtractor 7.
The subtractor 7 yields the luminance signal only by (Y+C)-C=Y. The luminance signal output goes to the output terminal 8.
Since the conventional Y/C separator is composed in the above manner, if the line correlation of the color carrier signal is extremely weak (for example, a change in the vertical direction in the picture such as from presence of color to absence of color), precise Y/C separation cannot be performed. Interference signals are therefore produced in the separated color carrier signal. Moreover, the same interference signals are produced in the separated luminance signal. Consequently, the problem of dot interference produced in the picture arises.